The present invention generally relates to a developing unit for an image forming apparatus implemented with an electrophotographic procedure and, more particularly, to an improvement in a developing roller included in the developing unit.
An electrophotographic copier, facsimile transceiver, printer or similar electrophotographic image forming apparatus has a developing unit for developing a latent image electrostatically formed on a photoconductive element. The developing unit has a casing loaded with a developer and a developing roller which transports the developer to the photoconductive element to develop the latent image as mentioned above. The developing unit is operable with a one-component developer or a two-component developer, as desired. The one-component developer is a magnetic toner and void of a carrier, while the two-component developer is a mixture of a non-magnetic or weakly magnetic toner and a magnetic carrier. The two-component developer is predominant over the one-component developer since it promotes easy control over the frictional charging of the toner and has desirable developing ability.
The developing roller has a rotary sleeve and a plurality of magnets which are so arranged in the sleeve as to face the inner wall of the sleeve. The magnets generate magnetic fields on the outer periphery of the sleeve to cause the developer to form a so-called magnet brush on the sleeve. The magnet brush constituted by the developer rises toward the photoconductive element with the result that the toner is electrostatically adhered to the latent image formed on the photoconductive element, thereby turning the latent image into a toner image.
The magnet brush is formed along the individual magnetic lines of force of the magnetic fields developed on the sleeve. Hence, the magnet brush rises substantially vertically from the central area of the pole-face of each magnet, but it begins to incline outward as it approaches the edges of the pole-face. The inclination sharply increases at the outside of the edge portions. Further, the magnet brush is inclined at axially opposite end portions of the sleeve in such a manner as to bulge out in the axial direction beyond the opposite end portions.
The magnets are arranged such that their poles alternate with each other. While the magnet brush is transported by the sleeve, it is effected by such magnets. The magnet securely positioned in the sleeve downstream of the others with respect to the transporting direction, i.e. a magnet having a collect pole removes the magnet brush remaining on the sleeve and collects it into the developing unit. However, the magnet brush is inclined outward at the axially opposite end portions of the sleeve, as mentioned above. The magnet brush, therefore, cannot be fully collected in the developing unit and is apt to fall into the interior of the image forming apparatus to thereby contaminate it.